Catalysis mediated by oxygen storage: HCl Oxidation Reaction over CeO2-x(111)-based Model Catalysts

通常，多相催化被认为是一种纯表面现象，其中离析物在催化剂表面转化为所需产物。然而，这种观点对于混合导电（电子和离子导电）的可还原氧化物来说太狭隘了，因为不仅电子与反应物交换，而且来自催化剂材料本体的氧离子也可以直接参与催化反应。将固态离子学与多相催化相关联的第一种方法使用储氧能力（OSC）的概念。事实上，固态离子学和多相催化之间的关系比OSC所能捕捉到的要复杂得多。氧缺陷化学影响催化剂的电子结构、活性位点的化学性质等等。在本项目中，催化和固态缺陷化学之间的密切关系将以技术上重要的多相催化反应为例，即通过CeO2催化剂的催化氧化从工业上无处不在的副产物HCl中回收氯。Deacon工艺允许在氯循环方面关闭工业过程。我们计划在合适的单晶CeO2（111）为基础的模型系统上进行表面化学和固态离子学实验。在密切的相互作用与从头算理论的基础上的密度泛函理论方法（DFT），反应机制将被阐明，特别侧重于在体和表面上的氧缺陷的作用。我们将开发一种CeO2（111）薄膜电化学电池，它允许通过电化学泵浦改变氧缺陷浓度，而与实际反应条件无关。与一个专门设计的间歇式反应器，我们将能够调查的影响，在散装和表面上的催化HCl氧化的动力学和确定的表面结构和Ce3+浓度与operando同步加速器为基础的方法的氧缺陷。特别地，可以鉴定催化活性相并且可以研究其在反应条件下的稳定性。

In general, heterogeneous catalysis is regarded as a pure surface phenomenon where the educts are converted to the desired product at the catalyst surface. However, this view is too narrow for mixed-conducting (electron and ion conduction) reducible oxides, since not only electrons are exchanged with the reactants, but also oxygen ions from the bulk of the catalyst material can participate directly in the catalytic reaction. A first approach to correlate solid state ionics with heterogeneous catalysis uses the concept of oxygen storage capacity (OSC). In fact, the relationships between solid state ionics and heterogeneous catalysis are more complex than what can be captured by OSC. The oxygen defect chemistry influences the electronic structure of the catalyst, the chemical nature of the active sites, and much more. In this project, the close relationship between catalysis and solid state defect chemistry will be exemplified with a technically important heterogeneous catalysed reaction, namely the recovery of chlorine from the industrially omnipresent by-product HCl via catalytic oxidation over CeO2 catalysts. The Deacon process allows industrial processes to be closed with regard to chlorine cycle. We plan to conduct experiments on surface chemistry and solid state ionics on suitable single-crystalline CeO2(111)-based model systems. In close interaction with ab-initio theory based on the density functional theory method (DFT), the reaction mechanism will be elucidated with special focus on the role of oxygen defects in the bulk and on the surface. We will develop a CeO2(111) thin film electrochemical cell, which allows to vary the oxygen defect concentration by electrochemical pumping, independent of the actual reaction conditions. With a specially designed batch reactor we will be able to investigate the influence of oxygen defects in the bulk and at the surface on the kinetics of the catalysed HCl oxidation and to determine the surface structure and Ce3+ concentration with operando synchrotron-based methods. In particular, the catalytically active phase can be identified and its stability under reaction conditions can be investigated.